1. Field of the Invention
The present invention generally relates to fault indicator circuits, and, more particularly, to a fault indicator circuit in a system having a plurality of circuit blocks each provided with an individual power source.
In a communication or information processing system having a plurality of circuit blocks each provided with an individual power source, if the individual power source of one of the circuit blocks is broken, it is necessary to indicate the fault occurrence by the use of power supplied from the individual power source of one of the other circuit blocks, so as to visually show which circuit block is out of order. Such fault indication is carried out by switching on a light-emitting diode of the broken circuit block, for instance.
2. Description of the Related Art
In a conventional communication or information processing system, a common power source which supplies power for a plurality of circuit blocks is generally employed, instead of individual power sources for the respective circuit blocks. A fault indication in using a common power source is carried out by the use of power from a primary source in a common power source unit (an input power source for driving the common power source). Since the primary source is a high-voltage power source, it is difficult to employ an IC. Instead of an IC, elements such as relays are used, resulting in a large-size circuit. It is difficult to carry out fault indication when individual power sources are power by one primary source.
Meanwhile, in a system provided with individual power sources, when a fault occurs in the individual power source of one of the circuit blocks, the circuit block needs to receive power from one of the other circuit blocks, because its own power source is no longer available. Therefore, the circuit block with the broken power source receives power from one of the other circuit blocks as well as a fault indication control signal. When a fault occurs in a component other than the individual power source, the circuit block with the broken component needs to indicate the fault occurrence on its own, because one of the other circuit blocks can supply power and the fault indication control signal only as long as the circuit block supplying power to the faulted circuit block is in a normal operation state. Accordingly, it is necessary to supply fault indication power from the faulted circuit block and one of the other circuit blocks. Also, two types of fault indication control signals are required: one is for indicating a fault in the individual power source, and is supplied from one of the other circuit blocks; and the other is for indicating a fault in a component other than the individual power source, and is supplied from the circuit block with the broken component itself. To drive an indication element such as a light-emitting diode (LED) with the two control signals, a method which uses open collectors as shown in FIG. 1 is conventionally employed.
FIG. 1 shows a circuit for transmitting fault indication control signals using open collectors. In this figure, OC1 and OC2 denote open collectors which are output gates for control signals. LED stands for a light-emitting diode. A fault indication signal for indicating a fault which cannot be detected and controlled by the circuit block (such as a fault in the individual power source) and a fault indication control signal which can be detected and controlled by the circuit block are inputted into the open collectors OC1 and OC2. respectively. When a control signal is generated, the light emitting diode LED emits light by means of power supplied from a power source 1 or a power source 2. When the individual power source of the power-supplying circuit block is broken, however, the open collector circuit that is the output gate for a control signal from the power-supplying circuit block does not stop as it is expected to (high impedance in the output state: HiZ). The power source for indicating fault occurrence might reach a certain voltage through a signal conductor from the power-supplying circuit block, because a CMOS (Complimentary MOS) is often used today due to the high-performance interface between circuit blocks. Here, the circuit blocks are printed boards, and the individual power source of each circuit block is called an on-board power source.
FIGS. 2 and 3 show the relationship between circuit blocks when a fault occurs in an individual power source.
FIG. 2 shows a first circuit block 81, a second circuit block 82 adjacent to the first circuit block 81, and a third circuit block 83. Arrows in the figure indicate power supply paths.
In a normal state, when a fault occurs in an internal component other than the power source of the first circuit block 81, a fault indication signal is inputted into the open collector circuit of the first circuit block 81 to switch on the light-emitting diode LED, thereby indicating the fault occurrence. However, when a fault occurs in the power source in the first circuit block 81, power is supplied from the second circuit block 82, via the paths indicated by the arrows in FIG. 2, to drive the open collector circuit and the light-emitting diode LED.
FIG. 3 shows the power supply paths in a case where a fault occurs in the individual power source in the second block 82. In this case, a small amount of current flows from the third circuit block 83 into the second circuit block 82 via control signal conductors, as indicated by arrows. As a result, a voltage is applied to the power source line of the second circuit block 82, though it is out of order. Accordingly, a voltage lower than a predetermined value is applied to each of gate circuits (1) and (2) for transmitting fault indication control signals. If the gate circuit (1) does not operate while the gate circuit (2) is in operation, the output of the gate circuit (1) becomes unstable. Receiving the unstable output, the gate circuit (2) makes a wrong judgment to switch on the light-emitting diode of the first circuit block, which is not broken.
A general object of the present invention is to provide fault indicator circuits, in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a fault indicator circuit which does not wrongly indicate a fault in a circuit block when the fault occurs in another circuit block in a system having a plurality of circuit blocks each provided with an individual power source. When a fault occurs in the individual power source of a circuit block in the system, the circuit block needs to indicate the fault occurrence by itself.
The above objects of the present invention are achieved by a fault indicator circuit which indicates that a fault has occurred in an individual power source in a system having a plurality of circuit blocks each provided with an individual power source, the fault indicator circuit comprising:
a fault indicator which indicates a fault occurrence in response to a fault indication control signal from a first circuit block or one of other circuit blocks, and which indicates, when a fault has occurred in the first circuit block, a fault occurrence in the first circuit block supplied with power from the individual power source of one of the other circuit blocks; and
an input determiner which is connected to an input terminal of a gate circuit into which the fault indication control signal from one of the other circuit blocks is inputted, and which lowers a level of each signal from an output circuit which outputs the fault indicator control signal from the other circuit blocks, thereby preventing the fault indicator of the first circuit block from being wrongly driven due to a fault in the individual power source of one of the other circuit blocks.
The above objects of the present invention are also achieved by a fault indicator circuit which indicates that a fault has occurred in a system having a plurality of circuit blocks each provided with an individual power source, comprising:
a fault indicator which indicates a fault occurrence in response to a fault indication control signal from a first circuit block of the circuit blocks or one of other circuit blocks, and which indicates, when a fault has occurred in the individual power source of the first circuit block, a fault occurrence in the first block supplied with power from the individual power source of one of the other circuit blocks; and
a voltage reducing unit which reduces a voltage and is located between a power source input terminal of a gate circuit into which the fault indication control signal from one of the other circuit blocks is inputted and a power source for driving a circuit which outputs the fault indication control signal from one of the other circuit blocks. With this fault indicator circuit, when a fault occurs in the individual power source of one of the other circuit blocks, the fault indicator of the first circuit block can be prevented from being wrongly driven.
In accordance with the present invention, wrong fault indication in a monitored circuit block can be prevented when a fault occurs in the power source of a circuit block monitoring the power source of the monitored circuit block. Thus, unnecessary maintenance operations, such as replacing a properly working circuit block with a new one, can be avoided.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.